The present invention relates generally to methods and apparatus for locating wireless transmitters, such as those used in analog or digital cellular systems, personnel communications systems (PCS), enhanced specialized mobile radios (ESMRs), and other types of wireless communications systems. This field is now generally known as wireless location, and has application for Wireless E9-1-1, fleet management, RF optimization, and other valuable applications.
Early work relating to the present invention has been described in U.S. Pat. No. 5,327,144, Jul. 5, 1994, xe2x80x9cCellular Telephone Location System,xe2x80x9d which discloses a system for locating cellular telephones using novel time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the ""144 patent are disclosed in U.S. Pat. No. 5,608,410, Mar. 4, 1997, xe2x80x9cSystem for Locating a Source of Bursty Transmissions.xe2x80x9d Both patents are owned by the assignee of the current invention, and both are incorporated herein by reference. The present inventors have continued to develop significant enhancements to the original inventive concepts and have developed techniques to further improve the accuracy of Wireless Location Systems while significantly reducing the cost of these systems.
Over the past few years, the cellular industry has increased the number of air interface protocols available for use by wireless telephones, increased the number of frequency bands in which wireless or mobile telephones may operate, and expanded the number of terms that refer or relate to mobile telephones to include xe2x80x9cpersonal communications servicesxe2x80x9d, xe2x80x9cwirelessxe2x80x9d, and others. The air interface protocols now include AMPS, N-AMPS, TDMA, CDMA, GSM, TACS, ESMR, and others. The changes in terminology and increases in the number of air interfaces do not change the basic principles and inventions discovered and enhanced by the inventors. However, in keeping with the current terminology of the industry, the inventors now call the system described herein a Wireless Location System. 
The inventors have conducted extensive experiments with the Wireless Location System technology disclosed herein to demonstrate both the viability and value of the technology. For example, several experiments were conducted during several months of 1995 and 1996 in the cities of Philadelphia and Baltimore to verify the system""s ability to mitigate multipath in large urban environments. Then, in 1996 the inventors constructed a system in Houston that was used to test the technology""s effectiveness in that area and its ability to interface directly with E9-1-1 systems. Then, in 1997, the system was tested in a 350 square mile area in New Jersey and was used to locate real 9-1-1 calls from real people in trouble. Since that time, the system test has been expanded to include 125 cell sites covering an area of over 2,000 square miles. During all of these tests, techniques discussed and disclosed herein were tested for effectiveness and further developed, and the system has been demonstrated to overcome the limitations of other approaches that have been proposed for locating wireless telephones. Indeed, as of December, 1998, no other wireless location system has been installed anywhere else in the world that is capable of locating live 9-1-1 callers. The innovation of the Wireless Location System disclosed herein has been acknowledged in the wireless industry by the extensive amount of media coverage given to the system""s capabilities, as well as by awards. For example, the prestigious Wireless Appy Award was granted to the system by the Cellular Telephone Industry Association in October, 1997, and the Christopher Columbus Fellowship Foundation and Discover Magazine found the Wireless Location System to be one of the top 4 innovations of 1998 out of 4,000 nominations submitted. The value and importance of the Wireless Location System has been acknowledged by the wireless communications industry. In June 1996, the Federal Communications Commission issued requirements for the wireless communications industry to deploy location systems for use in locating wireless 9-1-1 callers, with a deadline of October 2001. The location of wireless E9-1-1 callers will save response time, save lives, and save enormous costs because of reduced use of emergency responses resources. In addition, numerous surveys and studies have concluded that various wireless applications, such as location sensitive billing, fleet management, and others, will have great commercial values in the coming years.
Background on Wireless Communications Systems
There are many different types of air interface protocols used for wireless communications systems. These protocols are used in different frequency bands, both in the U.S. and internationally. The frequency band does not impact the Wireless Location System""s effectiveness at locating wireless telephones.
All air interface protocols use two types of xe2x80x9cchannelsxe2x80x9d. The first type includes control channels that are used for conveying information about the wireless telephone or transmitter, for initiating or terminating calls, or for transferring bursty data. For example, some types of short messaging services transfer data over the control channel. In different air interfaces, control channels are known by different terminology, but the use of the control channels in each air interface is similar. Control channels generally have identifying information about the wireless telephone or transmitter contained in the transmission.
The second type includes voice channels that are typically used for conveying voice communications over the air interface. These channels are only used after a call has been set up using the control channels. Voice channels will typically use dedicated resources within the wireless communications system whereas control channels will use shared resources. This distinction will generally make the use of control channels for wireless location purposes more cost effective than the use of voice channels, although there are some applications for which regular location on the voice channel is desired. Voice channels generally do not have identifying information about the wireless telephone or transmitter in the transmission. Some of the differences in the air interface protocols are discussed below:
AMPSxe2x80x94This is the original air interface protocol used for cellular communications in the U.S. In the AMPS system, separate dedicated channels are assigned for use by control channels (RCC). According to the TIA/EIA Standard IS-553A, every control channel block must begin at cellular channel 333 or 334, but the block may be of variable length. In the U.S., by convention, the AMPS control channel block is 21 channels wide, but the use of a 26-channel block is also known. A reverse voice channel (RVC) may occupy any channel that is not assigned to a control channel. The control channel modulation is FSK (frequency shift keying), while the voice channels are modulated using FM (frequency modulation).
N-AMPSxe2x80x94This air interface is an expansion of the AMPS air interface protocol, and is defined in EIA/TIA standard IS-88. The control channels are substantially the same as for AMPS, however, the voice channels are different. The voice channels occupy less than 10 KHz of bandwidth, versus the 30 KHz used for AMPS, and the modulation is FM.
TDMAxe2x80x94This interface is also known D-AMPS, and is defined in EIA/TIA standard IS-136. This air interface is characterized by the use of both frequency and time separation. Control channels are known as Digital Control Channels (DCCH) and are transmitted in bursts in timeslots assigned for use by DCCH. Unlike AMPS, DCCH may be assigned anywhere in the frequency band, although there are generally some frequency assignments that are more attractive than others based upon the use of probability blocks. Voice channels are known as Digital Traffic Channels (DTC). DCCH and DTC may occupy the same frequency assignments, but not the same timeslot assignment in a given frequency assignment. DCCH and DTC use the same modulation scheme, known as xcfx80/4 DQPSK (differential quadrature phase shift keying). In the cellular band, a carrier may use both the AMPS and TDMA protocols, as long as the frequency assignments for each protocol are kept separated.
CDMAxe2x80x94This air interface is defined by EIA/TIA standard IS-95A. This air interface is characterized by the use of both frequency and code separation. However, because adjacent cell sites may use the same frequency sets, CDMA is also characterized by very careful power control. This careful power control leads to a situation known to those skilled in the art as the near-far problem, which makes wireless location difficult for most approaches to function properly. Control channels are known as Access Channels, and voice channels are known as Traffic Channels. Access and Traffic Channels may share the same frequency band, but are separated by code. Access and Traffic Channels use the same modulation scheme, known as OQPSK.
GSMxe2x80x94This air interface is defined by the international standard Global System for Mobile Communications. Like TDMA, GSM is characterized by the use of both frequency and time separation. The channel bandwidth is 200 KHz, which is wider than the 30 KHz used for TDMA. Control channels are known as Standalone Dedicated Control Channels (SDCCH), and are transmitted in bursts in timeslots assigned for use by SDCCH. SDCCH may be assigned anywhere in the frequency band. Voice channels are known as Traffic Channels (TCH). SDCCH and TCH may occupy the same frequency assignments, but not the same timeslot assignment in a given frequency assignment. SDCCH and TCH use the same modulation scheme, known as GMSK.
Within this specification the reference to any one of the air interfaces shall automatically refer to all of the air interfaces, unless specified otherwise. Additionally, a reference to control channels or voice channels shall refer to all types of control or voice channels, whatever the preferred terminology for a particular air interface. Finally, there are many more types of air interfaces used throughout the world, and there is no intent to exclude any air interface from the inventive concepts described within this specification. Indeed, those skilled in the art will recognize other interfaces used elsewhere are derivatives of or similar in class to those described above.
The preferred embodiments of the inventions disclosed herein have many advantages over other techniques for locating wireless telephones. For example, some of these other techniques involve adding GPS functionality to telephones, which requires that significant changes be made to the telephones. The preferred embodiments disclosed herein do not require any changes to wireless telephones, and so they can be used in connection with the current installed base of over 65 million wireless telephones in the U.S. and 250 million wireless telephones worldwide.
Accordingly, a primary object of the present invention is to provide methods and apparatus for calibrating a wireless location system (WLS) to enable the system to make highly accurate time difference of arrival (TDOA) and frequency difference of arrival (FDOA) measurements. In a presently preferred embodiment of the invention, the instrumentation error is reduced by a calibration process whereby multiple wireless transmitters, such as cellular telephones, are placed at known locations throughout the coverage territory of the wireless location system. These phones make transmissions, such as periodic registrations or page responses, in a manner similar to any other phone. However, because their location and the theoretical TDOA values for any pair of SCS""s are known a priori, the TLP 12 can determine the exact error in the TDOA measurements made in connection with a particular pair of SCS""s. In addition, because the phones are in fixed locations and there is no Doppler shift, the theoretical FDOA value is zero. Any measured error will be due to drifts in the oscillators at each of the SCS""s, changes in the characteristics of analog components (e.g., the antennas, cabling, and filters), and environmental factors such as multipath. Rather than attempting to dynamically alter these individual error sources, which would introduce additional phase noise into the system, the external calibration method of the present invention corrects the computed TDOA and FDOA values in the digital signal processing stages of the SCS""s and TLP""s, which does not introduce such phase noise.
An external calibration method in accordance with the present invention comprises the steps of transmitting a first reference signal from a reference transmitter; receiving the first reference signal at first and second receiver systems; determining a first error value by comparing a measured TDOA (and/or FDOA) value with a theoretical TDOA (or FDOA) value associated with the known locations of the receiver systems and the known location of the reference transmitter; and utilizing the first error value to correct subsequent TDOA (or FDOA) measurements associated with a mobile transmitter to be located. A preferred implementation of this method further includes transmitting a second reference signal from a second reference transmitter; receiving the second reference signal at the first and second receiver systems; determining a second error value by comparing a second measured TDOA (or FDOA) value with a second theoretical TDOA (or FDOA) value associated with the known locations of the receiver systems and the known location of the second reference transmitter, and utilizing the second error value in combination with the first error value to correct subsequent TDOA (or FDOA) measurements associated with the mobile transmitter to be located. The first and second error values are preferably combined in a weighted average.
In presently preferred embodiments of the external calibration aspect of the invention, the error values are stored in tabular form for each baseline in the location system; the error values are combined in a time series weighted averaging method prior to being used to correct subsequent TDOA measurements; and the time series weighted averaging method is based on a Kalman filter. Preferably, the error values are weighted by a quality factor prior to being used to correct subsequent TDOA measurements, wherein the quality factor is based upon the output of a cross-correlation function of a reference signal received by the first and second receivers, and the error values are used by the location system only if the quality factor exceeds a prescribed threshold value. In addition, in preferred embodiments the location system monitors the rate of change of the error values and changes the rate of calibration, or time interval between calibrations, to ensure that the calibration rate exceeds the rate of change of the error values. The rate of calibration may be controlled, e.g., by automatically paging the reference transmitters.
An internal calibration method in accordance with the present invention is utilized to calibrate a first receiver system within an SCS, wherein the first receiver system is characterized by a time- and frequency-varying transfer function. The transfer function defines how the amplitude and phase of a received signal will be altered by the first receiver system, and the accuracy of a location estimate is dependent, in part, upon the accuracy of time measurements made by the receiver systems. The inventive method comprises the steps of injecting an internally generated wideband signal with known and stable signal characteristics into the first receiver system; utilizing the generated wideband signal to obtain an estimate of the manner in which the transfer function varies across the bandwidth of the first receiver system; and utilizing the estimate to mitigate the effects of the variation of the first transfer function on the time and frequency measurements made by the first receiver system. One such example of a stable wideband signal used for internal calibration is known as a comb signal, which is comprised of multiple individual frequency elements of equal amplitude and at a known spacing, such as 5 KHz.
In presently preferred embodiments of the internal calibration aspect of the invention, the estimate of the manner in which the transfer function varies across the bandwidth of the first receiver system is weighted by a quality factor prior to being used to mitigate the effects of the transfer function. The quality factor may be based upon the output of a cross-correlation function of the internally generated calibration signal and the same signal after it has passed through the transfer function. In addition, the antenna is first isolated from the receiver system prior to the injection of the internally generated calibration signal. An electronically controlled RF relay is preferably used to automatically isolate the antenna from the receiver system.
Other features and advantages of the invention are disclosed below.